The invention relates to security devices for use on documents of value, and in particular for use on banknotes and travellers' checks.
There is a continuing need to prevent the counterfeiting of valuable documents such as banknotes, checks, travellers checks, share certificates, stamps, plastic cash, credit and charge cards and the like. There is also a need for there to be facile authentication of the documents.
The continuing improvement in commercial printing and photocopying technology particularly requires that more advanced security features have to be created.
While machine readable features provide a useful line of defense, it is often useful to employ visual features or effects which can be noticed by a large number of people while the document is in use. This is particularly so for banknotes and other monetary instruments.
Embossing technology, for example as used in intaglio printing banknotes, has proved to be particularly useful in minimizing counterfeiting relative to other printing techniques. All banknotes contain some intaglio printing which may be additional to security printing applied by other techniques such as offset lithography. In practice the intaglio printing is undertaken as the last stage of printing except that numbers may subsequently be added, typically by letterpress printing.
It is known to undertake "blind embossing" where the intaglio plate only confers a surface profile to the substrate, without any ink being applied, for example as disclosed in U.S. Pat. No. 4,715,623.
An intaglio imprint is obtained by pressing a suitable substrate against an engraved and inked plate so as to deform the medium into the ink-bearing recesses of the plate. The resultant impression constituting a pattern formed of spaced and raised ink lines with optionally other pattern elements is characterised by superior clarity and sharpness of appearance and cannot be copied with sufficient fidelity to escape expert detection.
Examples of embossed, latent images are shown in U.S. Pat. No. 4,033,059, U.S. Pat. No. 4,715,623 and EP-A-194042. Such transitory images are generally difficult to see and are occasionally discrete, hidden, visual features.